Stability issue straight line…. or not

Were these burned in the orientation shown in the photos using a horizontal scan? The second photo with the material test makes me think you ran this vertically.

In any case, this seems most likely to be a loose belt issue. Recheck tension or for damage to belts/gears. Also check that the pinion gears are well secured and that they’re not slipping on the stepper shaft.

If not something mechanical then it’s possible you’re attempting to run too quickly and motors are skipping. I tend to think this isn’t the case since the material test shows problems even at low speeds.